There are a number of theories designed to explain why do we yawn. The most common theory is that yawning is a result of boredom or tiredness. So why do you yawn when you're tired? Scientists suggest that when you’re sleepy, tired, or bored, your breathing slows down and as a result, the required amount of oxygen is not produced in the body that is necessary for its functioning. In addition, slow breathing increases the levels of carbon dioxide gas in the body. As a means of coping, the brain triggers the yawning action.
A yawn is characterized by a wide opening of the mouth and the dropping of one’s jaw. This allows the maximum amount of air to be taken in by the lungs. This is followed by the slow exhalation of air alongside the flexing of abdominal muscles, the expanding of eardrums, and the pushing down of the diaphragm. When the brain needs a pick-me-upper, it will induce actions that trigger the long inhalation, filling the lungs with their full air capacity, dispelling the lungs of stale air, increasing your heart rate, and reviving of your senses